Support racks of this type are usually provided with several, vertically stacked, horizontal rows or tiers of protruding, article-engaging hooks upon which workers hang parts to be finished. The article engaging portion of the hooks may be constructed in a variety of shapes. Ordinarily each hook is simply a piece of wire having its article-engaging end bent into a hook shape. The term hook is utilized, however, to include the great variety of configurations which may be devised for engaging workpieces.
Conventionally, support racks are suspended from a conveyer and a large number of parts are connected to the hooks of the rack. The rack and its suspended workpieces travel through a finishing system having a series of stations at which they may be sequentially treated by one or more of the steps of cleaning, rinsing, surface preparation such as undercoating, rinsing again, drying, coating and baking. Usually finishing includes travel through a spray booth where the workpieces are sprayed and coated with a conventional material. They may thereafter advance through a baking oven in which the applied coating is heat treated, hardened and dried.
Workers then remove the finished parts and reuse the racks. However, because the racks are constantly recycled through the finishing system, they become encrusted with multiple layers of the coating material.
In electrostatic painting, the coating particles are electrically charged and directed toward a suspended workpiece. The workpiece is electrically connected to the ground potential through the conductive support rack so that the coating material is attracted by an electric field to the suspended parts. It is extremely important that the electrical contact between each support hook and the main frame of the support rack be maintained as a good, low resistance electrical contact so that the suspended parts will remain well grounded. The accumulation of nonconductive or high resistance coating particles at the interfacing surfaces between each hook and the main frame of the rack must be prevented.
If the coating material is applied to these contacting, interfacing surfaces, it will create a high resistance in the circuit from ground to the part which will inhibit electrical current flow. The current flow is necessary in order to supply electrons for neutralizing the charge of the coating particles after they are deposited on the suspended parts so that subsequently arriving, positively charged particles will be uniformily attracted to the workpieces. The undesirable resistance would ultimately result in a buildup of electrical charge in the area of the parts being painted upon the support racks. The buildup of charge would reduce the attraction of the part to the coating material and would also create a potential spark hazard caused by an arcing discharge which could ignite an explosion or fire.
A single painting or coating operation applies a coating layer to the exposed, article-engaging portion of the support hooks. This layer usually entirely covers the hooks except for the small area of direct contact with the part. Often a hook can be reused for an identical part which will be in electrical contact with the hook at the identical place. However, for a part which is positioned differently on the hook even one layer of coating material reduces electrical contact of the hooks with the workpieces.
It is therefore desirable that the hooks of the support rack be detachably engaged to the main frame of the rack so that the hooks may be periodically removed, cleaned and replaced. Alternatively, they may be removed and replaced with new hooks or with ones of a different shape to accomodate different parts.
One workpiece supporting rack which is shown in the prior art is described in U.S. Pat. No. 4,097,359 and the patents referred to therein. Still another work holder is shown in U.S. Pat. No. 2,904,492.
A major difficulty with the prior art racks is the difficulty of removing the hooks from the main frame of the rack. Another disadvantage is that the prior art racks are more expensive to construct because they require more metal and more manufacturing operations than the rack of the present invention. Because of the manner that the hooks are attached to the main frame of prior art racks, they have a further disadvantage that the wire size used for the hooks of a particular prior art rack is narrowly limited to the particular wire size for which the rack was designed.
It is therefore an object of the present invention to provide a versatile finishing support rack which has hooks which are removable or replaceable with a minimum of human manipulation and with hooks of a broad range of wire sizes and yet provides a finishing support rack which is of a stronger and sturdier construction and easier to fabricate than prior art racks and still provides complete shielding of the electrical contacts between the hooks and the main frame of the support rack.